The Champion, with 2 12" speakers, will project the sound more, so appear to be louder.What kind of music are you playing? How many players in the band? Although these amps are described as 100 watts, that's 100 watts solid-state and won't come close to the volume of a 50 watt Marshall tube amp, for example.The driven high-gain sound from the Fender amps has always been described as fizzy, the Champion seems to have gotten better from the reviews I have read.I think the Mustang has more FX and stuff built-in, hence its higher price.

I don't know much about the Champion, but I can vouch for the Mustang III being a very good amp for the price. I have gigged with it several times (both micing it into the PA and without a mic) and have never struggled to compete with the rest of the band's volume. Also, all of the effects are built in, which some people seem to dislike, but I find very helpful. You don't have to use the effects, but they do tend to be very helpful from time to time, and in the long run can save some money. And about the fizzing, I often see reviews with people complaining about this problem, but on my amp I have personally never had this problem unless I roll the master volume past 7 or so, which there's rarely a need for.